The installations presently used which are carrying out this type of treatment are generally made as continuous lines or batch furnaces. In said installations, the heating and the thermochemical treatment are carried out in the same chamber, the quenching being effected by immersion in a vat connected to the furnace via a lock. The parts to be treated are placed on plates, in baskets or on belts which represent often 50% of the total charge of the furnace.
The known installations are notably suffering from the following requirements and disadvantages:
1. a dispersion of the quality of the treated parts, due to the difficulty of obtaining homogeneity of the temperature and distribution of the active gas in the charge;
2. the length of the cycles, for the enrichments of reduced thickness, being tied to the use of conventional heating means, by convection-radiation, and to the research of a minimum of homogenity;
3. when operation incidents occur, a risk of having the entire charge or the contents of the furnace rejected, as the case may be;
4. high explosion hazards, tied to the size of the furnaces, to the atmospheres used and to the quenching vats;
5. the production costs are on the one hand increased due to the overconsumption necessitated by the joint heating of the baskets, plates or belts and the mass of the furnaces refractories, and on the other hand to the cyclical renewal of said conditioning materials made of noble alloys;
6. a lack of flexibility of the installations, the furnaces being oversized for the heaviest and bulkiest charges and having therefore a surplus capacity and being ill adapted to charges which are different from the thermal and aerodynamic points of view;
7. a difficulty of reducing, where continuous lines are involved, the quality dispersion and the length of the cycles, the parts which are preheated and those which are at the stage of diffusion being in the same enclosure.
These requirements and disadvantages, and the fact that such installations should be conducted by a specialized personnel, are such that it cannot be easily envisaged to integrate them in machining lines for the manufacture of large series of mechanical parts.